An application of an organic semiconductor to a photoelectric conversion element such as a solar cell, a light-emitting element, or a photosensor is expected. In particular, using a high molecular compound as an organic semiconductor material makes it possible to fabricate an active layer by a low-cost coating method. From a viewpoint of energy requirement and an emission reduction of CO2, a solar cell is expected as one of clean energies with a small environmental load, and a demand for this is rapidly increasing. Currently, a silicon-based solar cell is prevailing, but its efficiency is around 15% and it is difficult to reduce its cost. As a solar cell that can be fabricated at low cost, a CdTe solar cell has also been known, but since it uses Cd being a harmful element, it is liable to cause an environmental problem. Under such circumstances, practical use of an organic thin-film solar cell and an organic/inorganic hybrid solar cell as a next-generation solar cell that costs low, has high energy conversion efficiency, and is harmless, is being expected.
For example, there is a strong demand for improving power generation efficiency of the organic thin-film solar cell in order to put the organic thin-film solar cell into practical use. In order to improve the power generation efficiency, improving an open-circuit voltage (Voc) is important. A value of the open-circuit voltage of the organic thin-film solar cell greatly depends on the combination of an electron donor and an electron acceptor, and it is required to optimize materials used for these. It has been known that the open-circuit voltage of the organic thin-film solar cell correlates with a difference between a highest occupied molecular orbital (HOMO) energy level of a p-type material and a lowest unoccupied molecular orbital (LUMO) energy level of an n-type material.
It is thought that, in an organic thin-film solar cell currently under development, fullerenes such as phenyl-C61-butyric acid methyl ester (PCBM) are most suitable as the n-type material. As the p-type material, a conjugate high polymer of polythiophene such as poly(3-hexylthiophene-2,5-diyl) (P3HT) is generally used. The open-circuit voltage (Voc) of the organic thin-film solar cell in which PCBM and P3HT are combined is low such as about 0.6 V and is not necessarily satisfactory in view of practical application. It has been reported that by using, as the p-type material, a polymer in which imide is ring-condensed with thiophene, the open-circuit voltage improves up to about 0.85 V, but a further improvement in the open-circuit voltage is required for the purpose of enhancing the practicality of the organic thin-film solar cell. Further, in addition to an improvement in the open-circuit voltage, an improvement in life is required in the organic thin-film solar cell. In order to improve the life, active materials excellent in thermal stability (donor and acceptor) are necessary.
Further, recently, studies of an organic/inorganic hybrid solar cell whose energy conversion efficiency has been improved by using an organic/inorganic mixed perovskite compound and an inorganic perovskite compound for a photoelectric conversion layer are underway. In the organic/inorganic hybrid solar cell, as a hole transport layer, polyarylamine, and 2,2′,7,7′-tetrakids(N,N-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) are used, and in order to achieve higher efficiency, dopant agents such as t-butylpyridine (TBP) and lithium bis(trifluorononanesulfonyl)imide (Li-TFSI) are used. However, the TBP is a liquid and the Li-TFSI is a hygroscopic substance, so that due to diffusion and dissipation of the TBP into the photoelectric conversion layer, which are caused by a rise in temperature, and due to water molecule absorption caused by deliquesce of the Li-TFSI, performance degradation occurs. This results in a cause for a decrease in the life of the organic/inorganic hybrid solar cell. There has been also proposed to use, as the hole transport layer, P3HT being a p-type material, but it is not possible to obtain sufficient power generation efficiency in this case.